A group of bright green sea slugs thought to feed themselves using photosynthesis have been found capable of surviving for months without food - and in the dark!

Species of sarcoglassan sea slugs store chloroplasts from the single-celled algae they eat. These chloroplasts continue to photosynthesise — leading to the slugs' green colour — and scientists had previously assumed the slugs had evolved the ability to feed themselves using photosynthesis to protect them from starvation if algae was unavailable.

But researchers from Germany working with two of these so-called solar-powered slug species — Elysia timida and Plakobranchus ocellatus — found that if their ability to photosynthesise was blocked by keeping them in the dark they survived just as well as those exposed to light. Scientists denied food to six specimens of Elysia timida and kept them in total darkness for 88 days and all of them survived. What's more, they all lost around the same amount of weight as unfed specimens that were kept in lit conditions.

It's now thought that instead of being used as a source of solar power by the slugs, the stored chloroplasts are actually a food reserve. They remain photosynthetically active in the slugs' bodies, ready to be tapped into and consumed later if food is scarce. And because they move so slowly, it can take them some time to move from one food source to another, so this kind of adaptation has probably become necessary to ensure their survival.